Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than traditionally used silicon (Si), germanium (Ge) or gallium arsenide (GaAs). Diamond provides a higher energy band gap, a higher breakdown voltage and a greater saturation velocity than these traditional semiconductor materials. These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using conventional Si, Ge, or GaAs.
Silicon as a semiconductor material is typically not used at temperatures higher than about 200.degree. C. and GaAs is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for Si (1.12 eV at ambient temperature) and GaAs (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase. In addition, diamond has a smaller neutron cross-section which reduces its degradation in radioactive environments, that is, diamond is a "radiation-hard" material.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and use of diamond for high temperature and radiation-hardened electronic devices. Consequently, the fabrication of rectifying contacts, that is, Schottky contacts, on diamond will play an important role in the development of future diamond-based devices.
It has been demonstrated previously that gold (Au) or tungsten (W) contacts on a diamond layer provide rectification at temperatures of up to 400.degree. C. Unfortunately, the adhesion of these layers to the diamond, particularly at high temperatures, is often poor. Other rectifying contacts are also known. For example, U.S. Pat. No. 4,982,243 to Nakahata et al. discloses a Schottky contact which includes a monocrystalline diamond substrate, an epitaxial monocrystalline diamond layer on the substrate, and a Schottky electrode layer formed on the diamond layer. The Schottky electrode layer has a preferred thickness of 0.05 microns to 5 microns and is preferably of tungsten, molybdenum, niobium, tantalum, aluminum, polycrystalline silicon, nickel, gold, platinum, tungsten carbide, tungsten silicide or molybdenum silicide. In addition, the Schottky electrode layer may be formed on the epitaxial diamond layer by vacuum evaporation, chemical vapor deposition (CVD), or plasma CVD. The diamond layer is epitaxially grown on the surface of the monocrystalline diamond substrate, which surface inclines at an angle of not greater than 10.degree. to the (100) plane. The surface of the substrate is polished to produce the required uniformity of the diamond substrate.
The prior art has been limited to rectifying operation of contacts at relatively low temperatures, and also limited by poor adhesion of the metal contact layer to the diamond at elevated temperatures. In addition, there is a need for better device performance characterized by lower reverse leakage current and higher breakdown voltage.